A cold cathode fluorescent light (CCFL) has been increasingly used as a backlight source of a liquid crystal display (LCD) monitor of a notebook PC and of an LCD for use with a TV set. Such CCFL has substantially the same high efficiency and long life as a usual hot cathode fluorescent light, without using a filament implemented in the hot cathode fluorescent light.
The CCFL requires a high startup voltage and a high operating voltage. For example, a startup voltage of about 1000 V and an operating voltage of about 600 V are required. These high ac voltages are generated from a dc power supply of, for example, a notebook PC and a liquid crystal TV set, using an inverter.
Conventionally, a Royer circuit has been used as an inverter for the CCFL. The Royer circuit comprises a saturable magnetic core transformer and a control transistor, and is adapted to undergo a self-sustaining oscillation because of nonlinear permeability of the saturable magnetic core and nonlinear current gain characteristic of the control transistor. The Royer circuit itself requires no external clock or driver circuit.
However, a Royer circuit is basically a constant voltage inverter, which cannot provide a constant output voltage if the input voltage thereto and/or the load current thereof varies. Hence, in order to maintain a constant input voltage to the Royer circuit, a regulator for supplying electric power to the Royer circuit is required. For this reason, besides the inverter utilizing a Royer circuit has low power inversion efficiency, it is difficult to miniaturize such inverter.
A CCFL inverter having improved power conversion efficiency has been disclosed (see for example Japanese Patent Early Publication H10-50489). This inverter comprises a first semiconductor switch connected in series with the primary winding of a transformer, a second semiconductor switch and a capacitor which are connected in series with each other and in parallel with the primary winding, and a coupling capacitor is connected in series with the secondary winding of the transformer and with the load. The primary current of the transformer is fed back to a control circuit for comparison with a reference voltage to establish a control signal, which signal is used to control on-off operation of a first and a second semiconductor switches to provide a predetermined ac power to the load.
A full bridge type CCFL inverter (also called H bridge type inverter) utilizing four semiconductor switches has been also proposed (see for example U.S. Pat. No. 6,259,615). This inverter utilizes a transformer having a primary winding connected to the output terminal of the H bridge via a resonant capacitor connected in series with the primary winding. The load is connected to the secondary winding of the transformer. Of the four semiconductor switches constituting the H bridge, a first set of two semiconductor switches establishes a current path in a first direction to the primary winding of the transformer and a second set of two semiconductor switches establishes a current path in a second direction to the primary winding. By feeding the secondary current back to a control circuit for comparison of the voltage indicative of the current with a reference voltage, a control signal having a fixed pulse-width and controlled relative pulse position is generated. The control signal is provided to the semiconductor switches of the H bridge to thereby regulate the power supplied to the load. Further, the voltage across the secondary winding of the transformer is detected to secure over-voltage protection.
In conventional inverters, when the CCFL is temporarily stopped, electricity to the control circuit is generally cut off by a run-stop signal, putting the control circuit into a standby state.
In the standby state, drive signals to the semiconductor switches of the inverter are stopped as the power to the control circuit is cut off. However, the gates of the semiconductor switches receiving the drive signals have capacitances, that prevent the semiconductor switches from being turned off instantly if the drive signals were stopped, thereby causing the current to continue to flow for a while. This current continues to flow until the electric charge on the capacitive gate of each semiconductor switch is completely discharged through a pull-down (or pull-up) resistor. Consequently, the magnitude of the resultant current can be a few times as large as the normal load current.
This excessively large load current flows through the CCFL every time the CCFL is stopped, imposing a strong stress on the CCFL and shortening the life of the CCFL.
It is, therefore, an object of the invention to provide an inverter having a semiconductor switch circuit in the primary winding of a transformer, the inverter capable of performing constant current control of a load through pulse-width modulation (PWM) control of the switches of the semiconductor switch circuit and of preventing over-current from flowing in the load when the inverter is put into a standby state. It is a still another object of the invention to provide a controller IC for use with such inverter.